已经实现了一下，效果如下：

******欢迎使用万年历******
请输入年：
2018
请输入月：
10
2018年10月的天数为31天

然后源代码在下面，复制main方法就可以运行了。

package demo;

import java.util.Scanner;

public class Main {
        //代码来自  http://www.sojson.com/question/18895413383025630562.html
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int year;
		int month;
		int day =0;
		boolean runnian;
		System.out.println("******欢迎使用万年历******");
		System.out.println("请输入年：");
		year = input.nextInt();
		System.out.println("请输入月：");
		month = input.nextInt();
		if (year % 400 == 0 || year % 4 == 0 && year % 100 != 0) {
			runnian = true;
		} else {
			runnian = false;
		}
		switch (month) {
		case 1:
		case 3:
		case 5:
		case 7:
		case 8:
		case 10:
		case 12:
			day = 31;
			break;
		case 2:
			if (runnian) {
				day = 29;
			} else {
				day = 28;
			}
			break;
		case 4:
		case 6:
		case 9:
		case 11:
			day = 30;
			break;
		default:
			System.out.println("输入信息错误！");
		}
		System.out.println(String.format("%s年%s月的天数为%s天", year,month,day));
	}
}